One type of dental prophylaxis angle includes an elbow-shaped tubular housing with a forward tubular arm that supports an output rotor, and with a perpendicular tubular leg that supports an input rotor. The rotors have gears that mesh at the intersection of the tubular arm and leg. U.S. Pat. No. 3,163,934 by Wiseman shows an early version of this type of prophylaxis angle, which required the manufacture and installation of separate bearing parts to maintain the axial positions of the two rotors.
U.S. Pat. No. 3,727,313 by Graham describes an advanced dental prophylaxis angle construction, wherein the gear at the forward end of the input rotor served to fix the axial position of the output rotor, so no separate bearing was required to hold in the output rotor. That design used a hinged bearing that was pivoted out of the way to allow installation of the input rotor, with the bearing then pivoted through a slot in the housing to lie behind a rearward shoulder of the input rotor to prevent axial movement of the input rotor. However, the bearing extended only 90.degree. about the axis of the input rotor, and therefore did not provide good support for the input rotor.
Another design for a molded dental prophylaxis angle used an elbow-shaped housing formed in two parts that each extended by 180.degree. about the axes of the rotors. The rotors were installed in a first of the housing parts, and a second housing part then was placed over the first parts and the housing parts were heat welded together. One disadvantage of this design was the relatively high cost of manufacturing. A dental prophylaxis angle which had plastic molded parts that could be readily assembled, which provided a shoulder extending 360.degree. about the rear end of the shoulder on the input rotor, and which formed accurate circular, or radial bearing portions for accurately supporting the rotors in rotation, would be of considerable value.